Purple Wave Petunia Seeds (Ground Cover Type). The original Waves are still the best seed-raised trailing petunias. They require no pinching and a single plant can spread to 1.5 m (4′) across and hugs the ground at only 15 cm (6″) tall. Although described as a “ground cover”, Wave Petunias will not tolerate foot traffic. Colourful purple 5-7.5 cm (2-3") blooms smoother the plant.